1. Field of the Invention
The present invention relates to an optical control device such as an optical phase modulator and an optical equalizer using the same. More particularly, the present invention relates to a technique for preventing occurrence of polarization dependency in an optical control device of an optical waveguide type.
2. Description of the Related Art
Conventionally, an optical phase modulator is known as one of optical control devices to perform a phase modulation on light propagated through an optical waveguide.
As shown in FIG. 1, the phase modulator is composed of an optical waveguide 11 formed on a substrate 10 made of lithium niobate and a phase modulator 12 of a phase modulation electrode 12a and a phase modulation electrode 12b which are provided on the optical waveguide 11. In the phase modulator, an electric field is generated by applying a voltage between the phase modulation electrodes 12a and 12b and is applied to the optical waveguide 11. The refractive index of the substrate is varied by electro-optical effect in a portion of the optical waveguide 11 to which the electric field is applied. Thus, the phase of the light propagated through the optical waveguide 11 is modulated.
By the way, in such a conventional phase modulator, a distribution of refractive indexes in the optical waveguide is asymmetrical, and a distribution of stresses is asymmetrical due to the warp of the substrate and the change in the refractive index due to stress change in the vicinity of the optical waveguide. For these reasons, the polarization dependency is caused.
As a technique for preventing the polarization dependency, for example, a technique disclosed in Japanese Laid Open Patent Application (JP-A-Showa 62-36631: reference 1) is known. In this technique, for the prevention of the polarization dependency, a waveguide type optical modulator uses a polarization separating device (PBS) to separate incident light into a TE mode light and a TM mode light. Then, after the TE mode light and the TM mode light are individually subjected to phase modulation, the phase-modulated lights are combined using the polarization separating device.
Also, Japanese Laid Open Patent Application (JP-A-Heisei 7-199035: reference 2) discloses an optical waveguide type polarization scrambler. For the prevention of the polarization dependency at a time of a scrambling operation, the optical waveguide type polarization scrambler is composed of a phase modulator provided in a straight portion of an optical waveguide formed on a dielectric crystal substrate, and a quarter wavelength plate provided behind the phase modulator.
However, the prevention of the polarization dependency is insufficient in the reference 1 and the reference 2.
In conjunction with the above description, an optical integrated circuit is disclosed in Japanese Laid Open Patent Application (JP-A-Showa 64-77002). In this reference, the optical integrated circuit is composed of a substrate, a single mode optical waveguide, and a stress applying film. The single mode optical waveguide is embedded in a clad layer arranged on the substrate and has a core portion with a light propagation function. The stress applying film is arranged on a predetermined portion of the clad layer and irreversibly changes the stress acting to the core portion by trimming to adjust a stress birefringence.
Also, a waveguide type optical modulator is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 5-93891). In this reference, the optical modulator is composed of a crystal substrate with electro-optical effect, an input optical waveguide, two phase shift optical waveguides, an output optical waveguide, modulation electrodes and at lease a pair of bias control electrodes. The two phase shift optical waveguides are obtained by dividing the input optical waveguide, and are connected to the output optical waveguide as one body. The modulation electrodes are arranged in the neighborhood of the two phase shift optical waveguides. The pair of bias control electrodes is arranged in the neighborhood of the two phase shift optical waveguides.
Also, a single mode optical waveguide with a stress release groove is disclosed in Japanese Examined Patent Application (JP-B2-Heisei 6-46245). In this reference, the optical integrated circuit is composed of a substrate, a single mode optical waveguide, and a stress release groove. The single mode optical waveguide is composed of a clad layer and a core section. The clad layer has a thermal expansion coefficient different from that of the substrate. The core section is formed in the clad layer, and provides stress birefringence due to the difference in thermal expansion coefficient between the substrate and the clad layer. The stress release groove is provided locally along the extending direction of the core section on the clad layer surface in the neighborhood of the core section to release the stress and to adjust the stress birefringence.
Also, an optical wavelength filter is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 6-67129). In this reference, an optical wavelength filter is composed of a Mach-Zehnder interferometer with first and second waveguides, which are provided on a substrate with electro-optical effect. A ratio of a refractive index change for first polarized light to a refractive index change for second polarized light, in which the refractive index changes are caused the electro-optical effect in the first waveguide, is same as a ratio of a light path length for the first polarized light to a light path length for the second polarized light between the first and second waveguides.
Also, an optical bias adjusting method is disclosed in Japanese Laid Open Patent Application (JP-A-Heisei 7-28006). In this reference, an optical waveguide is formed on an optical crystal substrate with electro-optical effect. The optical waveguide has a refractive index larger than that of the substrate. A light transmissive film is formed on a part or parts of the optical waveguide by a deposition method or a sputtering method. By changing the refractive index due to the warp in the part or parts of the optical waveguide, the phase of light propagated through the optical waveguide is changed.
Therefore, an object of the present invention is to provide an optical phase modulator which can carry out phase modulation to a light signal while preventing polarization dependency.
Therefore, an object of the present invention is to provide an optical equalizer using the above phase modulator.
In an aspect of the present invention, an optical phase modulator includes a substrate having electro-optical effect. An optical waveguide is formed in a surface portion of the substrate. A phase modulation section is provided on the optical waveguide at least on an input side to carry out phase modulation to input light. An adjustment section is provided on an output side to eliminate polarization dependency of the phase-modulated light.
The phase modulation section may include a first phase modulation electrode provided on the input side, and a second phase modulation electrode provided on the output side. The phase modulation is carried out to the light by first electric field which is generated by applying a first voltage between the first and second phase modulation electrodes.
Also, the adjustment section may include adjustment electrodes provided on either side of the optical waveguide along the optical waveguide. The polarization dependency is eliminated by a second field which is generated by applying a second voltage between the adjustment electrodes.
Also, the phase modulation section may carry out the phase modulation to the light by changing a refractive index of the optical waveguide in response to a first voltage. The adjustment section may carry out the elimination of the polarization dependency of the phase-modulated light by changing a refractive index of the optical waveguide in response to a second voltage. In this case, a refractive index change rate in the phase modulation section and a refractive index change rate in the adjustment section may have different signs each other.
Also, the substrate may be formed of lithium niobate (LiNbO3) and cut out to have a plane orthogonal to an X-axis, and the optical waveguide may be formed in the surface portion of the substrate having the plane orthogonal to the X-axis and extends in a Z-axis direction.
In another aspect of the present invention, an optical phase modulator includes a substrate having electro-optical effect. An optical waveguide is formed in a surface portion of the substrate. A phase modulation section is provided on the substrate along the optical waveguide on an input side to carry out phase modulation to input light. An adjustment section is provided on an output side to eliminate polarization dependency of the phase-modulated light.
Also, the phase modulation section may include a first set of phase modulation electrodes provided on the substrate along the optical waveguide on one side of the optical waveguide, and a second set of phase modulation electrodes provided on the substrate along the optical waveguide. The phase modulation section may carry out the phase modulation to the light by first electric field which is generated by applying a first voltage between the phase modulation electrodes of the first set and second electric field which is generated by applying a second voltage between the phase modulation electrodes of the second set.
Also, the adjustment section may include adjustment electrodes provided on either side of the optical waveguide along the optical waveguide. The polarization dependency is eliminated by third electric field which is generated by applying a third voltage between the adjustment electrodes.
Also, the phase modulation section may carry out the phase modulation to the light by changing a refractive index of the optical waveguide in response to first and second voltages. The adjustment section may carry out the elimination of the polarization dependency of the phase-modulated light by changing a refractive index of the optical waveguide in response to a third voltage.
Also, a refractive index change rate in the phase modulation section and a refractive index change rate in the adjustment section have different signs each other.
Also, the substrate may be formed of lithium niobate (LiNbO3) and cut out to have a plane orthogonal to an X-axis. The optical waveguide maybe formed in the surface portion of the substrate having the plane orthogonal to the X-axis and extends in a Z-axis direction.
In another aspect of the present invention, an optical phase modulator includes a substrate having electro-optical effect. An optical waveguide is formed in a surface portion of the substrate. A phase modulation section is provided on an input side to change a refractive index of the optical waveguide for phase modulation to input light. An adjustment section is provided on an output side to change a refractive index of the optical waveguide for elimination of polarization dependency of the phase-modulated light. In this case, the refractive index change rate in the phase modulation section and the refractive index change rate in the adjustment section may have different signs each other.
In another aspect of the present invention, an optical phase modulator includes a substrate having electro-optical effect. Each of A plurality of phase modulation units are formed on the substrate includes an optical waveguide formed in a surface portion of the substrate. A phase modulation section is provided on an input side to change a refractive index of the optical waveguide for phase modulation to input light. An adjustment section is provided on an output side to change a refractive index of the optical waveguide for elimination of polarization dependency of the phase-modulated light.
Also, the refractive index change rate in the phase modulation section and the refractive index change rate in the adjustment section may have different signs each other.
In another aspect of the present invention, an optical equalizer includes a circulator which receives multiple wavelength light and outputs phase-modulated multiple wavelength light. An optical splitting and combining unit separates the multiple wavelength light supplied from the circulator into a plurality of single wavelength lights, and combines a plurality of single wavelength reflected lights into the phase-modulated multiple wavelength light to output to the circulator. A phase modulator carries out phase modulation to the plurality of single wavelength lights without polarization dependency to produce a plurality of single wavelength phase-modulated lights, and feeds back the plurality of single wavelength phase-modulated lights to the optical splitting and combining unit as the plurality of single wavelength reflected lights.
Here, the phase modulator may include a substrate having electro-optical effect, a plurality of phase modulation units and a mirror. The plurality of phase modulation units are formed on the substrate to carry out the phase modulation to the plurality of single wavelength lights without polarization dependency, and to pass the plurality of single wavelength phase-modulated lights. A mirror is provided to reflect the plurality of single wavelength phase-modulated lights such that the plurality of single wavelength phase-modulated lights pass through the plurality of phase modulation units and are supplied to the optical splitting and combining unit as the plurality of single wavelength reflected lights.
Also, each of the plurality of phase modulation units may include an optical waveguide formed in a surface portion of the substrate. A phase modulation section is provided on an input side to change a refractive index of the optical waveguide for phase modulation to input light. An adjustment section is provided on an output side to change a refractive index of the optical waveguide for elimination of polarization dependency of the phase-modulated light.
Also, the refractive index change rate in the phase modulation section and the refractive index change rate in the adjustment section may have different signs each other.
Also, the phase modulation section may include a first phase modulation electrode provided on the input side, and a second phase modulation electrode provided on the output side. The phase modulation is carried out to the light by first electric field which is generated by applying a first voltage between the first and second phase modulation electrodes.
Also, the phase modulation section may include a first set of phase modulation electrodes provided on the substrate along the optical waveguide on one side of the optical waveguide, and a second set of phase modulation electrodes provided on the substrate along the optical waveguide on the other side of the optical waveguide. The phase modulation section may carry out the phase modulation to the light by first electric field which is generated by applying a first voltage between the phase modulation electrodes of the first set and second electric field which is generated by applying a second voltage between the phase modulation electrodes of the second set.
Also, the adjustment section may include adjustment electrodes provided on either side of the optical waveguide along the optical waveguide. The polarization dependency is eliminated by third electric field which is generated by applying a third voltage between the adjustment electrodes.